High molecular weight polyesters are commonly produced from low molecular weight polyesters of the same composition by solid state polymerization. The low molecular weight polyesters which are used in such solid state polymerizations are prepared by conventional melt polymerizations. Solid state polymerization is generally considered advantageous in that the handling of high molecular weight ultra-high viscosity molten polymers during the polymerization phase is eliminated. Thermal degradation during the solid state portion of the polymerization is also essentially avoided.
In melt polymerizations, the major part of the reaction is transesterification due to the fact that thermal pyrolysis nullifies much of the esterification of carboxyl end groups. A much larger percentage of the reaction in a solid state polymerization is esterification. Thus, a higher molecular weight can be obtained in a solid state polymerization with a lower carboxyl concentration than can be achieved in a melt polymerization.
In the solid state polymerization of a polyester the polymerization is carried out at an elevated temperature which is below the melting point of the polyester resin. This polymerization is normally conducted in the presence of a stream of inert gas. The inert gas stream serves to remove volatile reaction products and helps to heat the polyester.